The invention relates generally to optical detectors. In particular, the invention relates to optical detection systems used in digital optical communication systems.
Communication networks, particularly those extending over long distances, are increasingly being implemented in optical fiber. At the transmitting end, a digital signal is impressed upon a laser light source typically outputting an optical signal in the 1.3 or 1.7 xcexcm wavelength band. The optical fiber, for example, a single-mode silica fiber waveguide, carries the digitally modulated optical carrier over long distances with little distortion. At the receiving end, an optical detector such as a photodiode receives the optical signal and outputs an electrical signal corresponding to data signal impressed on the transmitting end. Data rates are limited primarily by the electronics at either end. Data rates of 40 gigabits per second (Gbs) are possible with a single optical carrier although some fielded systems operate at no more than about 50 or 100 megabits per second (Mbs).
Although the optical loss in the optical fiber is relatively low, allowing transmission over tens to even hundreds of kilometers, eventually the signal attenuates to the point that it requires some sort of amplification. Amplification is also required because optical signals are sometimes split between multiple paths or the amplitude of the optical signal needs to be increased for detection or other purposes. One possible type of amplifier is a regenerator which detects the optical signal and uses the resultant electrical signal to control an optical transmitter which brings the optical signal back to an unattenuated level. However, in a more convenient and typically less expensive approach, an optical amplifier amplifies the optical carrier signal including any data modulation impressed upon it. Optical amplifiers are particularly advantageous in wavelength-division multiplexed systems in which a single fiber carries multiple optical carrier signals of slightly different respective wavelengths modulated with multiple respective data signals. A single well designed optical amplifier can amplify all of the wavelength channels while a separate regenerator needs to be provided for each wavelength channel.
The most prevalent optical amplifier is an erbium-doped fiber amplifier (EDFA), which has a fairly flat gain bandwidth of about 40 nm, which is sufficient for many WDM networks. However, optical amplifiers tend to be sensitive to variations in the average power levels on the fiber and to noise spikes. For these reasons, a robust network often includes a monitoring system located at an optical amplifier that determines the average power level and other physical parameters of the optical signals. Such a monitor taps a small amount of optical power from the signal on the cable and measures its intensity at a sampling rate of typically kilohertz to megahertz, far below the gigahertz rate of data signals.
Typically, the light is detected by a photodiode included in the monitoring system. However, photodiodes present some difficulties in measuring optical intensity to the significant resolution desired for the monitoring system. A photodiode is a light-sensitive electrical device, specifically a semiconductor diode which is negatively biased so that the photodiode passes very little current in the absence of light. The current in the absence of light is called the dark current. However, when the reversed biased photodiode is irradiated with light, such as that received from the optical fiber, the light generates electron-hole pairs, which are detected by the electronic circuitry as a current signal. In the presence of light, the signal includes both the photocurrent resulting from the incident light and dark current, of which the dark current is independent of the data signal. Because the photocurrent is being used to monitor the fiber and its value is not fixed, the dark current represents noise signal detracting from the accuracy of the measurement even though the noise signal varies only slowly with time.
Often the dark current can be reduced by the use of better components. However, better components may be excessively expensive, or the system performance is pushed even further so that dark current again becomes a problem. An alternative approach attempts to compensate for dark current and other similar sources of noise by determining their magnitudes and removing their contributions from the measured signal. Of course, in a commercial system, compensation must be accomplished without unduly complicating the system or excessively increasing its cost. Accurate power measurements when the dark current is of the same order of magnitude as the photocurrent requires compensation for the dark current. The typical tradeoffs are usually between the magnitude of the dark current, sensitivity of the photodiode, and the overall speed of the circuit.
Dark current is known to strongly depend upon the temperature of the photodiode. For this and other reasons, the photodiode is usually maintained at a fairly constant temperature. But temperature can be controlled only to a limited degree in a fielded commercial environment, and some temperature variations must be accepted. Furthermore, the photodiode is subject to some uncontrollable aging effects which may affect the dark current. Particularly the temperature variations limit the sensitivity of the system.
For these reasons, it is desirable to provide dynamic means for compensating the dark current of a photodiode, that is, to periodically, over periods of minutes to hours, readjust the detection circuitry to account for the change in dark current.
A system and associated method determine the effect of photodiode dark current or other DC offsets in a monitoring system associated with an optical communication channel which uses a photodiode detecting a signal tapped from the channel to determine characteristics of the channel and its signal, for example, average signal power or noise spikes. A known signal is passed through the photodiode, and its detected value is extracted to allow the photodiode or other electrical circuitry to be characterized.
The known signal may be a locally generated repetitive signal with a known harmonic signature or characteristic and is combined with the tapped signal. Signal processing including harmonic analysis allows the detected signal to be separated into the channel component and the component arising from the known signal.
Optionally, the tapped signal may be selectively attenuated so that during the attenuation period mostly the known signal is being detected.
Alternatively, the tapped signal is nearly completely attenuated during a system characterization period so that the known signal corresponds to a dark signal being received. No locally generated harmonic signal is required.
Advantageously, the system includes a temperature sensor and storage means for storing system response values at a number of temperatures while the tapped signal is attenuated. When the tapped signal is not so attenuated, the temperature is measured and the one or more of the stored system response values is used to correct the system response value according to the measured temperature. The system can be calibrated while the tapped signal is attenuated by controllably heating the system and recording the system response associated with the temperature.